Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Buy an item from the Camp Store on Christmas.
Complete Meat Circus.
Revisit all brains after completion.
Achieve Rank 101.
Gather all Figments.
Sort all Emotional Baggage.
Crack all Vaults.
Complete The Asylum.
Find Edgar's Secret Garden.
Introduce all Camp Kids to Mr. Pokeylope.
See yourself through the eyes of many others.
Re-brain the Children.
Use the bacon. A lot.
Achieve Rank 80.
Redeem all 16 Scavenger Hunt Items.
Complete Waterloo World.
Find all Golden Helmets.
Complete Black Velvetopia.
Earn all Merit Badges.
Complete Gloria’s Theater.
Complete The Milkman Conspiracy.
Achieve Rank 60.
Complete Lungfishopolis.
Achieve Rank 40.
Cook and consume two different kinds of roast in one sitting.
Spy on Bobby's love life.
Give the Coach's speech on the stump.
Hear Vernon's Ghost Story.
Complete Milla’s Dance Party.
Uncover Milla's Secret.
Achieve Rank 20.
Read many bulletin board messages.
Discover the secret history of Whispering Rock.
Witness Maloof's transformation.
Complete Sasha’s Shooting Gallery.
Complete Basic Braining.
Complete the Punchy Target mini-game.